1. Field of the Invention
The present invention relates to a magnetic carrier to be used in an image forming method for developing an electrostatic charge image by use of electrophotography and a two-component developer using the magnetic carrier.
2. Description of the Related Art
In a conventional electrophotographic-system image forming method generally employed, an electrostatic latent image is formed on an electrostatic latent image bearing member by use of various processes and toner is adhered to the electrostatic latent image to develop the image. In developing the image, a carrier particle called a magnetic carrier is mixed with toner to triboelectrically charge the toner. In this manner, an appropriate amount of positive or negative charge is imparted to the toner. The toner is developed by using the charge as driving force. This is a two-component development system, which has been widely used.
In the two-component development system, since a magnetic carrier can play a part in stirring, transporting and charging of a developer, the function of the magnetic carrier is clearly distinguished from that of a toner. This is advantageous since the performance of the developer can be easily controlled.
However, with technological evolution of the electrophotographic field, it has recently been more and more strenuously demanded to not only reduce the space and volume of a main body of an apparatus but also increase the operation speed and extension of life of the apparatus, as well as high definition and stable quality of image.
In the circumstances, an attempt has been made to reduce the size and the number of parts of a main-body apparatus and save energy power. Also in development, it is demanded to reduce the size of a transformer. If the strength of a development electric field is increased, a flying amount of toner and uniformity of a solid image and a half-tone image can be improved; however, adhesion and leakage of a carrier tend to occur, causing image defects. Because of this, in order to stably develop an image even in a low electric field, an attempt has been made to improve the developing performance of a magnetic carrier.
Furthermore, it is required for a developer to provide stable developing performance for a long time. To obtain long-term stability, an attempt has been made to reduce specific gravity and magnetic force of a magnetic carrier. Employing ferrite using a light element, porous ferrite and a magnetic substance-dispersed resin carrier has been proposed. A magnetic carrier prepared by filling and coating a porous magnetic ferrite with a resin and defining the strength of an electric field right before breakdown of the magnetic carrier is proposed (International Application No. WO2010/016605). As a magnetic substance-dispersed resin carrier, a magnetic substance-dispersed resin carrier obtained by direct polymerization of spherical magnetite and phenol is proposed (Japanese Patent No. 2738734).
Furthermore, with an improvement of low temperature fixation of toner, even in these carriers, adhesion or fusion (so called toner spent) of toner to a carrier surface is a matter of concern.
To deal with the problem, a magnetic carrier preventing toner spent and peel-off and wear of a coating layer, attaining long-term stability is proposed (Japanese Patent Application Laid-Open No. 2011-013676). In the carrier proposed herein, the irregularity of the carrier core surface ascribed to the shape of magnetite having a large particle diameter is controlled by varying the shape of magnetite different in size. By virtue of this, the adhesiveness of a coating layer is improved and peel-off and wear thereof are reduced, improving durability. However, if the shape of magnetite changes from a spherical shape to an irregular shape, the degree of shape anisotropy of a magnetic substance increases and residual magnetization increases. When such magnetite is used, the carrier sometimes takes the form of a chain and adheres to a region to which toner particles should be jumped and attached. This phenomenon is particularly significantly observed if the resistance of a carrier core is reduced in order to improve developing performance in a low electric field. As a result of the chain like carrier adhering to a solid image portion, the carrier serves as a spacer during transferring, reducing a transfer electric field. The toner around the carrier is not transferred, creating “fog”, with the result that the solid image loses uniformity.
The mechanism of causing carrier adhesion when a low-resistant carrier core is used is conceivably as follows. Since the performance of a carrier using a low-resistant carrier core to charge a toner is low, the counter charge of the carrier is also low. When the carrier having a low counter charge is charged by developing bias applied to a development sleeve, the charge polarity of the carrier surface is reversed and charged with the same polarity as that of a toner, with the result that the carrier jumps and attaches to an image portion.